Process for the treatment of liquefied hydrocarbons using 3-(piperazine-1-YL)propane-1,2-diol compounds

The invention relates generally to processes for the treatment of liquefied hydrocarbons. More specifically, the invention relates to processes for removing acid gases from liquefied hydrobarbon gas streams such as liquefied petroleum gas (LPG) or natural gas liquids (NGL) using piperazine compounds.

Liquefied hydrocarbons such as NGL or LPG, present a flammable mixture of hydrocarbon gases used as a fuel in heating appliances and vehicles. It is increasingly used as an aerosol propellant and a refrigerant, replacing chlorofluorocarbons in an effort to reduce damage to the ozone layer.

Liquefied hydrocarbons are synthesized by refining petroleum or “wet” natural gas, and are almost entirely derived from fossil fuel sources, being manufactured during the refining of petroleum (crude oil), or extracted from petroleum or natural gas streams as they emerge from the ground.

Liquefied hydrocarbons may evaporate quickly at normal temperatures and pressures and may be supplied in pressurized steel gas cylinders. These cylinders are typically filled to between 80% and 85% of their capacity to allow for thermal expansion of the contained liquid. The ratio between the volumes of the vaporized gas and the liquefied gas varies depending on composition, pressure, and temperature, but is typically around 250:1.

Liquefied hydrocarbons often contain a variety of acidic, gaseous contaminants, such as hydrogen sulfide, a variety of mercaptans and other diverse sulfur compounds, carbon dioxide, and carbonyl sulfide (COS). It is well known in the gas treating industry that such contaminants can be successfully removed by contacting gas or liquid hydrocarbon streams with aqueous solutions of one or more amines. Aqueous amine solutions may be either selective or non-selective in their ability to absorb particular acid gases.

After such absorption, the acidic compounds are stripped from the amines and the amines are returned to the system, except to the extent that the amine compounds may have been lost in the process. It has been theorized that many different amines would provide some level of utility for removal of acid gases. As a practical matter, the amines actually in commercial use are monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), and diisopropanolamine (DIPA).

Treatment of liquefied hydrocarbons presents particular problems in that amines tend to be significantly soluble in the liquefied hydrocarbons, leading to a corresponding economic penalty due to the need to make up the lost amine(s). Many refineries use aqueous DIPA or MDEA to remove the acidic impurities from liquefied hydrocarbons. However, the concentration of these amines is typically limited to the range of about 20-35 weight percent of the aqueous stream in which they are supplied to the process. Operation at higher concentrations, which is desirable for capacity reasons, generally results in undesirably high levels of liquefied hydrocarbons contamination with amine(s).

The problem is particularly acute at refineries treating cracked (i.e., highly unsaturated) LPG. Often, the loss rate of MDEA is sufficient to negate the economic justification for substituting MDEA for DEA.

All of U.S. Pat. Nos. 5,326,385; 5,877,386; and 6,344,949 teach some type “sweetening” of LPG through various processes. More specifically, U.S. Pat. No. 5,877,386 teaches the use of mixtures of triethanolamine with other amine species. Further, U.S. Pat. No. 4,959,086 uses isomers of amine compounds to remove hydrogen sulfide from natural gas. Use of MDEA/DIPA mixtures has also been reported (U.S. Pat. No. 4,808,765) for the purpose of removing H 2 S.

These publications present reasonable solutions to problems encountered when “sweetening” liquefied hydrocarbons through the amine-acid gas processes. However, it would be highly desirable to have an amine composition which maximizes the effective amine concentration circulating in the liquefied hydrocarbons system, while yet minimizes the amount of amine(s) lost due to solubility in the liquefied hydrocarbons.

In accordance with one aspect of the invention, there is provided a method for treating liquefied hydrocarbons comprising acid gases to remove the acid gases while minimizing loss of amine species. The method comprises the step of contacting the liquefied hydrocarbons with an absorbent aqueous solution of a first amine compound, the first amine compound having the structure:

wherein R 1 is hydrogen or propane-2,3-diol and R 2 is propane-2,3-diol, or mixtures thereof.

When aqueous solutions of traditional alkanolamines such as methyldiethanolamine (MDEA) are used to treat liquefied petroleum, gas within liquid/liquid processes, important amine losses can be encountered over time. The presence of hydroxyl groups has proved to be critical in reducing these losses by improving the lipophobic character of the molecule. Therefore, triethanolamine (TEA), incorporating three hydroxyl groups, remains the molecule of choice even though aqueous solutions of MDEA proved to be superior to aqueous solutions of TEA in terms of performance and capacity for acid gas removal. The difference in performance and capacity between MDEA and TEA is mainly dictated by the difference in basic strength reflected by their respective pKa of 8.7 for MDEA and 7.9 for TEA.

Therefore, alkanolamine structures incorporating an increased number of hydroxyl groups and/or nitrogen-hydrogen bonds compared to MDEA while maintaining a low molecular weight along with a basic strength (i.e. pKa) equal or superior to TEA would be ideal candidates for treating liquefied petroleum gas within liquid/liquid processes.

The incorporation of propanediol moieties into alkanolamine structures allows for reduced solubility in hydrocarbon streams compared to equivalent alkanolamine structures incorporating hydroxyethyl moiety (i.e. traditional ethoxylated alkanolamines). The basic strength of alkanolamine incorporating propanediol moieties is not altered compared to traditional ethoxylated alkanolamines since inductive effects engendered by the presence of more than one hydroxyl group on the same substituent of nitrogen do not cumulate. Moreover, most of these structures can be reached by the simple reaction between glycidol epoxide or 3-chloro-1,2-propanediol with piperazine or substituted piperazine derivatives as seen below.

For purposes of this disclosure, liquefied hydrocarbons are those low molecular weight hydrocarbons which may be saturated or unsaturated, branched or unbranched ranging in size from about C 1 to C 20 , preferably from about C 1 to C 12 , more preferably from about C 2 -C 6 such as for example, LPG or NGL, or mixtures thereof.

FIG. 1 is a graphical illustration of the relative solubility of the tested amines compared to MDEA plotted against their pKa values.

Generally, the invention is a method for treating liquefied hydrocarbons comprising the removal of acid gases while minimizing loss of amine species. The method comprises the step of contacting the liquefied hydrocarbons with an absorbent aqueous solution of a first amine compound, the first amine compound having the structure

wherein R 1 is hydrogen or propane-2,3-diol and R 2 is propane-2,3-diol, or mixtures thereof.

A principal disadvantage of the amines commonly used in the prior art is their relativity high solubility in LPG. The invention addresses that problem by providing an amine compound with a lower LPG solubility.

Most refineries operate at a total amine concentration of no more than about 35 weight % of the amine containing, aqueous treatment composition. Operation at about 40 weight %, preferably even about 50 weight % total amine(s) or more is desirable since high strength solutions provide additional gas removal capacity at low cost. Also, it is likely that the concentration of sulfur in crude oil will rise in the future.

Accordingly, in order to maintain or increase production, the refinery must, on the average, process/remove more sulfur. Nevertheless, because of the increased loss of amines at the higher concentrations, it has not been economically feasible to operate above about the 35% level in most cases. One advantage of the invention is that it allows the refinery to operate economically at higher total amine strengths without the high amine replacement costs they would otherwise incur.

In accordance with the invention, there is provided a method of removing acid gas from liquefied hydrocarbon gas. The method relies on an aqueous solution of amine compounds of the formula:

wherein R 1 is hydrogen or propane-2,3-diol and R 2 is propane-2,3-diol, or mixtures thereof.

The incorporation of propanediol moieties into alkanolamine structures allows for reduced solubility in liquid hydrocarbon streams compared to equivalent ethoxylated alkanolamine structures. The basic strength of alkanolamine incorporating propanediol moieties is not altered compared to traditional ethoxylated alkanolamines since inductive effects engendered by the presence of more than one hydroxyl group on the same substituent of a nitrogen do not cumulate. Moreover, most of these structures can be reached by the simple reaction between glycidol epoxide or 3-chloro-1,2-propanediol with piperazine or substituted piperazine derivatives.

Generally, the first amine in the process of the invention may comprise a piperazine amine with one or more propanediol functionality. Representative piperazine compounds among others include:

Compounds such as these, as listed above, may be used individually or in mixture to comprise the first amine to sweeten or otherwise remove acidic gases from the untreated LPG. Generally, the first amine compound may be synthesized through any number of means known to those of skill in the art.

In addition to the first amine compound used in the process of the invention, the aqueous solution used to sweeten LPG may comprise a second amine compound. Amine compounds useful as the second amine compound include trisamine compounds such as 2-amino-2-(hydroxymethyl)propane-1,3-diol, 2-methylamino-2-(hydroxymethyl)propane-1,3-diol, 2-dimethylamino-2-(hydroxymethyl)propane-1,3-diol, or mixtures thereof; amine propanediol compounds such as 3-(2-(hydroxyethyl)methylamino)propane-1,2-diol, 3-(methylamino)bis(propane-1,2-diol), amino-tris(propane-1,2-diol), 3-(methylamino)propane-1,2-diol, 3-(amino)propane-1,2-diol, 3-(amino)bis(propane-1,2-diol) or mixtures thereof; alkyl amines such as monoethanolamine, diethanolamine, triethanolamine, methyldiethanolamine, diisopropananolamine, and mixtures thereof; and mixtures of compounds within each of these species heretofore listed above.

Method of Treatment

The process of this invention may be readily implemented by contacting a liquefied hydrocarbon stream such as NGL, LPG, or mixture thereof with the aqueous mixtures of the invention using ordinary liquid-liquid contacting equipment, and under operating conditions within the ordinary limitations of such equipment. While some optimization of conditions, within the skill of the art, should preferably be done, it is to be expected that a reduction in amine solubility losses will be experienced even at existing operating conditions. A further advantage of the invention, therefore, is that it does not require significant substitutions or modifications in equipment, packing, operating conditions, and the like. Accordingly, the present invention is particularly beneficial to refineries which need more acid gas removal capacity, but are reluctant to pay for extensive capital upgrades.

It is another advantage of this invention that operating parameters are not narrowly critical. As a general guideline, it may be said that the higher the concentration in the system, the higher will be the amine losses. Representative concentrations are found below. While there is not known specific upper limit on concentration, it is suggested that the concentration be held to no more than about 95 weight % of the amine mixture, the remaining being water, in order to avoid operational problems, such as inadequate removal of H 2 S. A useful approach to determining the maximum usable concentration of in a given system is to gradually increase the content until problems are detected, then back off on the concentration until such problems disappear.

Similarly, there is no necessary minimum concentration, this concentration may be a matter of routine experimentation. It is suggested, however, as a starting point that the concentration be at least about 5 weight %. It is believed that, in the majority of cases, the useful range of concentrations will be about 10 to about 90 weight %, preferably about 25 to about 75 weight %, and more preferably about 35 to about 65 weight % of the amine mixture, the remaining being water.

The aqueous absorbant solution used in the method of the invention may also comprise an acid such as boric acid, sulfuric acid, hydrochloric acid, phosphoric acid, and mixtures thereof. The concentration of acid may vary in an amount effective from 0.1 to 25 weight % and most preferably from 0.1 to 12 weight %. The addition of acid is helpful in recovering the amine composition after the acid gas is stripped from the system.

The operating temperature for the contacting of the LPG with the containing amine mixture is not narrowly critical, but will usually be in the range of about 50° F. to about 190° F., preferably about 70° F. to about 160° F., and more preferably about 80° F. to about 140° F. In general terms, the lower temperatures are preferred in order to minimize solubility losses. Since most refineries do not have much flexibility in this regard, it is an advantage of this invention that significant reduction in amine loss will be effected at any given operating temperature.

Working Examples

The following examples provide a non-limiting illustration of the features of the invention.

A solution of heptane (10 g), toluene (0.1 g) and the tested amine (2.5 g) are mixed at 20° C. for 1 hour. The mixture is decanted for 15 minutes and the neat heptane phase is analyzed by gas chromatography using toluene as internal standard. The injection is repeated three times and peak areas of tested amine are averaged (HEP stand for 2-(hydroxyethyl)piperazine). Results are presented below:

The pKa of the tested amines was recorded using an automated Mettler Toledo titration system using 50 weight % aqueous amine solutions and 0.5 N hydrochloric acid. Results are presented below:

Although the present invention has been described by reference to its preferred embodiment as is disclosed in the specification and drawings above, many more embodiments of the present invention are possible without departing from the invention. Thus, the scope of the invention should be limited only by the appended claims.